Rendering with CUDA NVidia RTX 2080 Super GPU same speed as CPU (3900x Ryzen)
Brainscattered answer:
Use RTX not CUDA if you want to see an improvement in render speed.
The CPU is never idle when rendering in GPU, there will always be activity.
If the scene uses only 8% of the GPU it means that the scene is not very complex, hardly a challenge for the graphics card. Blender will not use more than what is needed. With a more complex scene and more samples, etc you will see the GPU work harder (Can you make the cpu go to 100% when typing in word or when reading email? )
In GPU one tile per GPU is rendered, whereas the CPU renders many tiles simultaneously, one per thread, so it is perfectly possible that a multicore-multithread CPU can be faster than GPU.
And forget about measuring CPU and GPU usage, those numbers say very little when it comes to the overall performance of blender.